1. Field of the Invention
The present invention relates to an analog digital conversion circuit that converts an analog signal to a digital signal.
Priority is claimed on Japanese Patent Application No. 2009-221775, filed Sep. 28, 2009, the content of which is incorporated herein by reference.
2. Description of the Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
Time A/D converters (TADs) are one kind of pulse-delay-type analog-to-digital (A/D) converter circuit. A time A/D converter includes an annular delay circuit in which a plurality of delay units (such as inverting circuits) is connected in an annular shape.
In conventional time A/D converters, a pulse current is caused to circulate around the annular delay circuit while an input voltage as a target of A/D conversion is supplied to the power supply terminals of the delay units that constitute the annular delay circuit. Conventional time A/D converter performs A/D-conversion of the input voltage based on the number of circulations of the pulse current per predetermined period of time and on the “voltage-delay time” characteristic. Here, the “voltage-delay time” characteristic refers to a characteristic in which the higher the input voltage value to the power supply terminal of the delay unit is, the shorter the propagation delay time of the pulse current is.
A time A/D converter that performs A/D-conversion of an input voltage based on the “current-delay time” characteristic instead of the “voltage-delay time” characteristic is disclosed in Japanese Unexamined Patent Application, First Publication No. 2003-32113. Here, the “current-delay time” characteristic refers to a characteristic in which the higher the input current value to the power supply terminal of the delay unit is, the shorter the propagation delay time of the pulse current is.
In the time A/D converter based on the “current-delay time” characteristic, current control elements (control transistors) are connected to the delay units of the annular delay circuit (pulse delay circuit) on a one-to-one basis (FIG. 3(c) of Japanese Unexamined Patent Application, First Publication No. 2003-32113). With the current control elements, the time A/D converter is capable of restricting a through current to suppress electric power consumption.